1. Field of the Invention
The present invention relates to an electronic musical instrument and more specifically to an analog delay line driven by two clock output signals of opposite phase provided by a voltage-controlled oscillator, the two clock output signals being of a frequency which is inversely proportional to the control voltage applied at the modulation input of the voltage-controlled oscillator, the analog delay line thereby providing delay which is directly proportional to the control voltage and eliminating distortion which arises when delay is a non-linear function of the control voltage.
2. Description of the Prior Art
Various types of voltage-controlled oscillator circuits for driving analog delay lines are described in the prior art. A two-phase, voltage-controlled oscillator, for example, is used as a clock in the tone modulation system of U.S. Pat. No. 4,031,795 issued to Bunger. Such two-phase, voltage-controlled oscillators are illustrated in blocks 12, 27 and 33 in FIG. 1 of the Bunger patent, where they are utilized to drive analog shift registers 11, 26 and 32, respectively. Voltage-controlled oscillators of various types are also used to provide clock signals for driving analog delay lines in U.S. Pat. No. 4,038,898--Kniepkamp et al; U.S. Pat. No. 3,833,752--van der Kooij; U.S. Pat. No. 3,866,505--Adachi; U.S. Pat. No. 4,043,243--Peterson; U.S. Pat. No. 4,080,861--Wholahan; U.S. Pat. No. 4,144,790--Suchoff; and U.S. Pat. No. 4,096,778--Dittmar.
The operation of previously known voltage-controlled oscillators for providing clock signals to drive analog delay lines differs fundamentally from the operation of the present invention in that the output frequency of prior art voltage-controlled oscillators is directly proportional to the control voltage applied. The present voltage-controlled oscillator circuit provides the reciprocal relationship between frequency and control voltage to that of prior art circuits; i.e., the present voltage-controlled oscillator circuit produces output signals whose period (rather than frequency) is directly proportional to the control voltage applied at the input of the circuit. The delay provided by an analog delay line is directly proportional to the period of the clock signals which drive it. Prior art tone modulation systems, such as that disclosed in the Bunger patent, introduce frequency distortion in the audio signal being delayed because of the non-linear relationship between delay and control voltage. This non-linear relationship results because the delay is inversely proportional to the frequency of the clock signal, while the frequency of the clock signal in prior art circuits is directly proportional to the control voltage; hence the delay is inversely proportional to the control voltage. The use of a period-proportional voltage-controlled oscillator in the present invention produces a more symmetrical modulation which sounds better, especially for large amounts of modulation. It also provides for a smoother string chorus modulation, and eliminates the need for a calibrating adjustment during production with a corresponding saving of labor.